Published: September 9, 2012 Reshaped and renamed by generations of developers and gentrifiers, the borders of New York City’s neighborhoods are often hazy at best. Yesterday’s Chinatown is today’s east TriBeCa; a resident of Bedford-Stuyvesant may, after some real estate alchemy, morph into a citizen of Clinton Hill.

These distinctions, with status, self-identity and resale values at stake, can often lead to heated disputes, so much so that a state assemblyman, Hakeem Jeffries of Brooklyn, introduced a bill, the Neighborhood Integrity Act, in 2011 to tamp down the tension. (The bill failed.) And City Hall offers little help: the city has never codified neighborhood boundaries, leaving profit-hungry brokers and civic activists to fight it out.

But now, thanks to the democratizing force of the Internet, dozens of amateur cartographers are reshaping these lines themselves, taking advantage of malleable Web sites — including Google Maps and Wikipedia — to provide their own definitions for where, for instance, Park Slope ends and Gowanus begins.

Their judgments are far-reaching: Google Maps, which provides user-generated outlines for every city neighborhood, is consulted much more often than any Rand McNally atlas. A result is a new class of unsung urban arbiters, empowered to turn one’s uber-hip NoHo apartment into just another East Village walk-up, for all the world to see.

“It is a lot of responsibility,” said Matthew Hyland, 31, a chef who lives in Downtown Brooklyn.

In his spare time, Mr. Hyland rejects or approves dozens of changes to Google’s online map of New York City, which received more than 2,000 proposed revisions last month from users of Google Map Maker. He consults city documents, community boards and local blogs before making any changes to a neighborhood’s boundaries, viewing himself as more umpire than activist. He avoids upstart terms peddled by brokers, as when he overruled a user who tried to relabel a swath of Brooklyn waterfront as “Rambo” — for Right Around Manhattan Bridge Overpass.

“I like my maps accurate,” Mr. Hyland said.

Ask the professionals, though, and they will question the very notion of a formal, down-to-the-intersection neighborhood grid, like the one presented by Google.

“Anyone who says there is a defined neighborhood is off his rocker,” said Lisa Keller, executive editor of The Encyclopedia of New York City, a meticulously researched tome of five-borough facts.

When she set out to define neighborhood borders for the book, Ms. Keller contacted dozens of local experts and historians for each entry. “I talked to 20 people and got 12 different answers,” she said. In the end, she relied on a rough consensus for the borders she used in the book.

Once, at a lecture, Ms. Keller asked the attendees which Brooklyn neighborhood the old Ebbets Field had been in.

“I thought there was going to be a fistfight in the audience,” she recalled. Google Map Maker, which became available in the United States last year, lets users submit revisions to any component of Google’s map. The changes are reviewed by a Google employee or an outside “regional expert reviewer,” like Mr. Hyland, who was selected by Google for his frequent and accurate contributions.

The position is unpaid, though Mr. Hyland said he had received a T-shirt. And his authority is not unlimited. “I’m not allowed to do something like delete the Lincoln Tunnel,” he said.

For online editors, some demarcations are easy. Few Brooklynites, for instance, would dispute that DeGraw Street marks the boundary between Cobble Hill and Carroll Gardens. (Those areas, however, are of recent vintage: until the 1950s, the brownstone blocks below Atlantic Avenue were simply called South Brooklyn.)

But a neighborhood that is still forming, or changing in complexion, can pose a challenge. Last year, Mr. Hyland extended the border of Gowanus, an industrial Brooklyn neighborhood gaining some residential traction, from Bond Street to Hoyt Street, a one-block incursion into neighboring Carroll Gardens.

Craig Hammerman, district manager at the community board that covers the area, which is home to a notoriously foul-smelling canal, said that he thought Bond Street would be a more appropriate boundary, but that he would defer to residents. “We don’t put up a fuss if someone wants to call themselves by a different name,” Mr. Hammerman said. “A rose is still a rose.”

But Mr. Hammerman, a 24-year resident, said he was alarmed that Google’s Gowanus did not include the Gowanus Houses, a housing project, which instead was in nearby Boerum Hill, where Wikipedia also put it. And Mr. Hyland, for his part, already claims restraint — he said he had so far resisted the impulse to eliminate Gowanus from the map altogether.

With its vast audience, Google’s online map has already caused some consternation. This summer, a Google contributor added the label “soccer field” to a portion of Fort Greene Park in Brooklyn, despite community efforts to banish soccer games there. The park’s conservancy called the label “definitely misleading.”

And the comment threads on Map Maker can turn hostile. In Manhattan, the borders of Hudson Square, around the Holland Tunnel, and of Turtle Bay, by the United Nations, prompt fierce debate. TriBeCa keeps creeping eastward, from the traditional Broadway to Lafayette or Centre Street. “That’s real estate people doing that kind of thing,” said Mr. Hyland, who rejects those proposed changes. “They want to sell a loft in TriBeCa instead of a loft in Chinatown.”

Barry F. Hersh, a professor at the Schack Institute of Real Estate at New York University, said online maps were only the latest tool in a long line of technology influencing geography. “The telephone company once decided which neighborhood you were in,” he said.

Mr. Hersh, who grew up near the Brooklyn Navy Yard, scoffed at that area’s relatively new name, “Vinegar Hill.”

“I can tell you, growing up there, I never heard that,” he said. Mr. Hersh suggested that an enterprising broker had simply decided the area needed a better name than what he used to call it: “Next to the sewage plant.”